1. Field of the Invention
The present invention relates to a method for dithering, and more particularly to a method for dithering that prevents distortion of an image and the degradation of image quality.
2. Description of the Related Art
In order to meet various requirements of users, the functions of digital image devices for reproducing colors (such as monitors, scanners, printers, etc.) have diversified, and have been improved. Also, according to the field of use of each digital image device, digital image devices may adopt different color spaces or color models according to their use fields. Color models are classified into device-dependent color models and device-independent color models. Examples of the device-dependent color models include red-green-blue (RGB) models, which are types of addition color space models, and cyan-magenta-yellow-black (CMYK) color models which are types of subtraction color space models. Examples of the device-independent color models include CIE color models, such as a CIE L*a*b* model, a CIE XYZ model, and a CIE LUV model, etc. The CIE color models are determined by the International Commission on Illumination (ICI) which determines standards for illumination devices. The CIE XYZ model represents a set of RGB tristimulus values as another set of tristimulus values which are all positive X, Y, and Z values. The CMYK color space is used in the field of printing, and the RGB color space is used in the field of graphic presentation, e.g., computer monitors, used for Internet input/output. When it is impossible to use a required color in representing an input image with high gradation on an output device with low gradation precision, a dithering technique is mainly used for representing similar colors by mixing a set of dots of colors with different gradations. For example, on display devices or printing devices, an image, or portions thereof, may be represented either in a gray color with a particular gradation according to the ratio of black dots to white dots, or in pink colors with various gradations according to the ratio of red dots to white dots. In an output device with a low resolution, the dithering technique is used to improve the reality of an image and prevent contours, etc., which are not smooth but rough, from catching the eyes of viewers. The conventional dithering technique as described above has been used to represent an input image signal with higher resolution than that of an output device. As conventional dithering technique, spatial dithering and/or spatial-temporal dithering are used.
The spatial dithering technique adopts a method for removing some lower bits from an image signal on an input-bit basis. Specifically, some lower bits are removed from an image signal on a bit-by-bit basis. Then, the spatial dithering technique generates a random number from a random number generator, compares a magnitude of the generated random number with a magnitude of the removed bits, and then selectively adds “1” to a Least Significant Bit (LSB) of an input image signal.
The dithering method using a random number as described above causes the problem that noise equivalent to a random number generated in the process of dithering is intactly inserted into either an area, where a particular Gray pattern appears, or a flat area with a single tone. Also, the generation of a random number is not uniform, but may be concentrated on a number. Therefore, for example, the degradation of image quality is generated such that colors in a particular area of an image become light or dark.
Hence, there has been a need for a image processing method which minimizes color distortion in the entire image, and in which the degradation of image quality caused by dithering does not appear even in a flat area of an image.